Re: [PATCH tty-next 14/22] tty: Remove tty_wait_until_sent_from_close()

[Peter Hurley]

On 06/17/2014 07:32 AM, Peter Hurley wrote:
> On 06/17/2014 07:03 AM, David Laight wrote:
> > From: Peter Hurley
> > ...
> > > > I don't understand the second half of the changelog, it doesn't seem
> > > > to fit here: there deadlock that we are trying to avoid here happens
> > > > when the *same* tty needs the lock to complete the function that
> > > > sends the pending data. I don't think we do still do that any more,
> > > > but it doesn't seem related to the tty lock being system-wide or not.
> > >
> > > The tty lock is not used in the i/o path; it's purpose is to
> > > mutually exclude state changes in open(), close() and hangup().
> > >
> > > The commit that added this [1] comments that _other_ ttys may wait
> > > for this tty to complete, and comments in the code note that this
> > > function should be removed when the system-wide tty mutex was removed
> > > (which happened with the commit noted in the changelog).
> >
> > What happens if another process tries to do a non-blocking open
> > while you are sleeping in close waiting for output to drain?
> >
> > Hopefully this returns before that data has drained.
>
> Good point.
>
> tty_open() should be trylocking both mutexes anyway in O_NONBLOCK.

Further, the tty lock should not be nested within the tty_mutex lock
in a reopen, regardless of O_NONBLOCK.

AFAICT, the tty_mutex in the reopen scenario is only protecting the
tty count bump of the linked tty (if the tty is a pty).

I think with some refactoring and returning with a tty reference held
from both tty_open_current_tty() and tty_driver_lookup_tty(), the tty
lock in tty_open() can be attempted without nesting in the tty_mutex.

Regardless, I'll be splitting this series and I'll be sure to cc
you all when I resubmit these changes (after testing).

Regards,
Peter Hurley




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